762/39 steelcase explosion

was this the only round that had an issue?

If yes, out of how many fired?

my money is on a single case failure, which happens

FWIW that ammo looks European. Do you have the whole case? What colour is the spam can?
 
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It happened twice over the course of shootimg about 200 round s. The spam cans were the typical dark green. All the cartridges were on stripper clips and had asian writing on slips of paper in each can. It was sold and advertised as chinese ammo from the dealer. Still waiting for the gunsmith to call.
 
kawicrash said:
Looks like a classic unsupported case head failure. Hopefully the gunsmith will get to the bottom of it and see what's up. I can't remember now, is this the only ammo you've used so far?
Click to expand...

Every AR15 has an unsupported case head.



The black line in this picture is the normal last point of support for an AR chamber. It was designed by Stoner to support the brass case only up to the thick part of the web to leave the base free to expand or rupture in the case of an over pressure such as a bore obstruction. The rear of AR barrels have a large radius at the rear to be a pressure relief valve.

7.62 x 39 mm surplus is steel cased and not necessarily suitable for a chamber with the last point of support so far forward.

The feed ramps have nothing to do with the chamber in an AR. The locking recess is between the ramps and the rear of the barrel.

In any case, you can clearly see the last point of support marked on the case that failed, meaning that the bolt was fully forward. If the firing pin is struck it means the bolt was locked except in the extremely rare case that the front part of the firing pin was broken and somehow jammed fully forward - but this is easy to eliminate as a possibility since the firing pin would be clearly broken. If the rifle were to fire with an unlocked bolt the case head is normally separated and the rest of the case remains in the chamber as the bolt slams rearward - usually damaging the receiver in the process.

Therefore case had to have been weak or damaged, or there was a moderate high pressure incident, possibly a bore obstruction. An obstruction nearly always bulges the barrel at the point of impact. If the case was flawed, i.e. too soft, there is no way to prove it positively since the metal is now damaged and brittle from the incident and can't be measured or hardness tested.

You can eliminate "unsupported case head failure" since there is no such thing in an AR. You can eliminate slam fire or out of battery ignition since the evidence shows the case was fully forward and locked. In the absence of evidence of a bore obstruction, that leaves only a flawed case.

Cheap, bulk Chinese ammo would be consistent with that conclusion.
 
Except I don't think it's Chinese... Chinese usually only has the plant and year stamped on the head.
Albanian maybe?
 
gunner2015 said:
It happened twice over the course of shootimg about 200 round s. The spam cans were the typical dark green. All the cartridges were on stripper clips and had asian writing on slips of paper in each can. It was sold and advertised as chinese ammo from the dealer. Still waiting for the gunsmith to call.
Click to expand...
Did the case have a seam on the top and the bottom and did you open it with a spam key, or did it have only a seam at the top and you opened it with a jumbo can opener?
 
Albanian had 3 military plants; guns, and amd explosives. I forget which was ammo. But I think we have a winner with the sloppy sealant being albanian
 
The other thing to check is the dimensions of the cartridge case. The rifle headspace may be fine, but if the ammo is excessively short, the same head separating criteria are present.
I imagine Albanian QC in the 80's wasn't real high...might be the pics, but the shoulders look crimped or fluted or something odd.
Edit...
With reference to the first firing pin not protruding enough to set this ammo off that has me thinking it was probably in spec, and the undersized ammo was what was the issue. Subsequently the longer firing pin allowed the shorter ammo to fire, creating the headspace headpops.
 
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Finally got back the upper.first of all the ammo was in green spam cans,there was no opener so i used tinsnips.it was all on stripper clips,the qc papers in each can were in asian,i suspect chinese as it was sold by the shop i bought it at as chinese ammo.the gunsmith said that the chamber was in spec.he figured the ammo was to small and when it was fired the steel case doesnt expand enough in the chamber after ignition and obviously blew out of the case.from his many many yrs of experience with ammo he advised that the 762/39 has been made in many countries around the world and some have slightly changed the measurements and there a difference as compared to sammi specs , which i would think that every rifle manufactured in canada would be made to the sammi specs. He also stated that some countries who were chrome lining barrels and chambers had some issue chrome lining the chamber corners and they changed the ammo very slightly to fit.i have a few test boxes of different ammo that iam going to try out.i hope that i explained this ok.so basically its an ammo issue of being to short in the chamber..whoooo
 
Albania was aligned with China in that period, probably why there was Chinese writing in or on it. They didn't trust the west or the Russians...they also made their own SKS rifles at one point, not sure they ever made AK's?
 
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